The person who first created a 5-string bass guitar, based on the clearest available records, is closely tied to the American instrument manufacturer Fender.
Specifically, the “Fender Bass V,” released by Fender in 1965, is widely recognized as the world’s first commercially produced 5-string bass. This instrument added a high “C” string to the traditional 4-string bass tuning (E-A-D-G), resulting in a configuration of E-A-D-G-C. At the heart of its development was likely Leo Fender, the founder of Fender, who is also renowned for popularizing the electric bass itself. It’s highly probable that the idea for a 5-string bass stemmed from his innovative vision.
However, it’s more reasonable to assume that rather than Leo Fender personally crafting it himself, the team at Fender, under his leadership, designed and produced it. By that time, Leo had sold Fender to CBS in 1965, but since the development of the Bass V took place just before this transition, his influence is believed to have been significant.
Historically, 5-string variants of the double bass existed prior to this, and while the concept of a 5-string instrument wasn’t entirely new, Fender is credited with being the first to bring a 5-string model to market as a modern electric bass. Therefore, the most likely name associated with “the person who first made a 5-string bass” is Leo Fender.
(image from https://ja.wikipedia.org/wiki/%E3%83%95%E3%82%A7%E3%83%B3%E3%83%80%E3%83%BC%E3%83%BB%E3%83%99%E3%83%BC%E3%82%B9V)
